Saccadic oscillations facilitate ocular perfusion from the avian pecten

THE evolution of the eye is constrained by two conflicting requirements--good vascular perfusion of the retina, and an optical path through the retina that is unobstructed by blood vessels. Birds are interesting in that they have higher metabolic rates and thicker retinas than mammals, but have no retinal blood vessels. Nutrients and oxygen must thus reach the neurons of the inner retina either from the choroid through 300 micron of metabolically very active retina, or from the pecten, a pleated vascular structure protruding from the head of the optic nerve into the vitreous chamber, and more than a centimetre away from some retinal neurons. Despite the diffusional distance involved, several lines of evidence indicate that the pecten is the primary source of nutrients for the inner retina: the presence of an oxygen gradient from pecten to retina, the large surface area produced by macroscopic folds and by microscopic infoldings of the luminal and external surfaces of the capillary endothelium, extrusion of circulating fluorescein, high content of carbonic anhydrase and alkaline phosphatase, and retinal impairments after pecten ablation. Another peculiarity of birds, their saccadic oscillations, occur with a large cyclotor-sional component during every saccadic eye movement. In different species, saccades, which occur at intervals of 0.5-40 s, have up to 13 oscillations with frequencies of 15-30 Hz and ampliá-tudes of about 10 degrees. Therefore, as much as 12% of some birds' total viewing time may be subject to the image instability caused by the oscillations. Using fluorescein angiography, we show here that during every saccade, the pecten acts as an agitator which propels perfusate towards the central retina much more effectively than is observed during intersaccadic intervals.     

Amino-acid synthesis in carbonaceous meteorites by aqueous alteration of polycyclic aromatic hydrocarbons

It has been suggested that amino acids and other organic compounds found in carbonaceous meteorites formed by aqueous alteration in the meteorite parent bodies. Observations of carbonaceous material in interstellar grains and interplanetary dust particles indicate that condensed organic compounds may have been present in meteorite parent bodies at the time of aqueous alteration. One group of compounds thought to be representative of this carbonaceous material is the polycyclic aromatic hydrocarbons (PAHs). Recently it was proposed that PAHs condense on SiC grains in the molecular envelopes of carbon-rich red-giant stars, which would allow for their subsequent incorporation into meteorite parent bodies during accretion. This incorporation mechanism is supported by the identification of SiC grains in carbonaceous chondrites. The possibility therefore exists that PAHs, and/or other condensed organic compounds, represent the starting material for aqueous alteration which leads to the formation of amino acids and other water-soluble organic compounds. Here we present calculations of the distribution of aqueous organic compounds in metastable equilibrium with representative PAHs as functions of the fugacities of O2, CO2 and NH3. The results reported here for pyrene and fluoranthene, two PAHs with different structures but the same stoichiometry, differ greatly but indicate that the formation of amino and carboxylic acids is energetically favourable at probable parent-body alteration conditions. The actual reaction mechanisms involved could be revealed by consideration of isotope data for PAHs, amino acids, other organic compounds and carbonates in carbonaceous chondrites.     

The in vivo expression of the globin genes of theβ cistron in γ-,δ-, andδβ-thalassemia heterozygotes

There is considerable evidence suggesting that the switch from to and chain production after birth is due, in part, to silencing of the genes by stage-specific factors which bind to their promoters and to the competition from the adult ( and ) genes for a common enhancer element located in the locus control region. As a consequence one can expect that the increased Hb F production in adults with hereditary persistence of fetal hemoglobin or -thalassemia is directed mainly by -globin genes in cis to the deletion(s) responsible for these conditions. Here we review data on heterozygotes with -, -, or -thalassemia, who also had an^AT mutation, in cis or in trans, which was used as a marker of gene expression. The results show that a deletion affecting adult genes favors the expression of genes in cis, while the deletion of a single gene does not affect the expression of the gene in cis but leads to a faster switch postnatally.     

Detection of toxic metabolites in the hemolymph ofBeauveria bassiana infectedSpodoptera exigua larvae

Highly active metabolites have been detected in the hemolymph of the lepidopteranSpodoptera exigua infected with the mycopathogen,Beauveria bassiana. A combination of phenyl sepharose and CM ion exchange chromatography was utilized to extract the active metabolites from infected hemolymph samples. The active in vivo metabolites, having a molecular mass greater than 10 KDa, were thermolabile and were inactivated by proteinase K. These metabolites were characterized by their ability to disrupt metamorphosis, killing treated larvae at the wandering or pupal stage. Additionally, injection ofS. exigua larvae with active samples caused a reduction in the number of filopodial-producing hemocytes. The biological activities and biochemical properties suggest that novel compounds are produced duringB. bassiana mycosis.     

Fibronectin peptide DRVPHSRNSIT and fibronectin receptor peptide DLYYLMDL arrest gastrulation ofRana pipiens

Gastrulation is characterized by dramatic cell migration which is thought to require the interaction of cell adhesion molecules with extracellular molecules. We have tested two novel peptides, a fibronectin peptide and a fibronectin receptor peptide, for their effects on gastrulation of the leopard frogRana pipiens. The fibronectin peptide DRVPHSRNSIT corresponds to residues 1373–1383 of the cell-binding domain of fibronectin; the receptor peptide DLYYLMDL corresponds to residues 124–131 of 1 subunit of a variety of integrins including 51. Either of these peptides significantly inhibited gastrulation after being microinjected into mid-blastulae. These results indicate that these sequences may correspond to the ligand/receptor interaction sites of fibronectin and its receptor(s).     

Atmospheric carbon dioxide concentrations before 2.2 billion years ago

The composition of the Earth's early atmosphere is a subject of continuing debate. In particular, it has been suggested that elevated concentrations of atmospheric carbon dioxide would have been necessary to maintain normal surface temperatures in the face of lower solar luminosity in early Earth history. Fossil weathering profiles, known as palaeosols, have provided semi-quantitative constraints on atmospheric oxygen partial pressure (pO2) before 2.2 Gyr ago. Here we use the same well studied palaeosols to constrain atmospheric pCO2 between 2.75 and 2.2 Gyr ago. The observation that iron lost from the tops of these profiles was reprecipitated lower down as iron silicate minerals, rather than as iron carbonate, indicates that atmospheric pCO2 must have been less than 10(-1.4) atm--about 100 times today's level of 360 p.p.m., and at least five times lower than that required in one-dimensional climate models to compensate for lower solar luminosity at 2.75 Gyr. Our results suggest that either the Earth's early climate was much more sensitive to increases in pCO2 than has been thought, or that one or more greenhouse gases other than CO2 contributed significantly to the atmosphere's radiative balance during the late Archaean and early Proterozoic eons.     

对数浓度图在氧化还原滴定分析中的应用

文中就对数浓度图在氧化还原滴定分析中的绘制及其应用进行了较为详细的讨论。     

反胶束溶液和血红蛋白水溶液相平衡的研究

研究了十六烷基三甲基溴化铵－正庚烷反胶束溶液和牛血红蛋白水溶液的相平衡,探讨了有机相助表面活性剂种类和浓度、水相ｐＨ值、水相离子种类和强度、表面活性剂浓度的影响,适宜条件下,牛血红蛋白的萃取率可达９５％以上。对实验结果从表面活性剂极性头与蛋白质分子间的相互作用和反胶团包溶蛋白质时的空间阻碍作用上进行了解释。     

苯甲醛缩氨基硫脲Cu(I)、Zn(Ⅱ)配合物的直接电化学合成

用电化学金属阳极氧化法在非水溶剂中合成了苯甲醛缩氨基硫脲（ＨＬ）与Ｃｕ（Ｉ）、Ｚｎ（Ⅱ）配合物,通过元素分析、红外光谱、紫外光谱、磁矩、摩尔电导等对配合物进行了表征。     

土法造纸木素降解之探索Ⅲ.嫩毛竹自然发酵过程中的细菌分离鉴定与嫩竹软化菌的筛选

试验证实,嫩毛竹自然发酵过程中的微生物主要是细菌。分离出并纯化了１００株细菌,其中６０株是芽胞杆菌。将分离出的１００株细菌分别接入石灰预处理过的嫩竹,从中筛选出能使嫩竹进一步软化的菌株３株,也均为芽胞杆菌。对这６０株芽胞杆菌进行了形态学及其生理生化特征的鉴定,按Ｐｒｉｅｓｔ氏等（１９８８年）的数值分类系统分类到组,将３株（３０８,３０９,３１０）对嫩竹软化有明显作用的芽胞杆菌的１株（３０８）鉴定到种,认为是短小芽胞杆菌（Ｂ．ｐｕｍｉｌｕｓ）。另外４０株非芽胞杆菌参阅《伯杰氏系统细菌学手册》第１卷和第２卷,鉴定到科或属,有微球菌、葡萄球菌、李氏杆菌、不动杆菌、肠杆菌科等。